Discus Prandtl tube

[Serge Serfaty]

On the vertical fin of the original Discus, there are
3 holes. one for securing the elevator, one where the
total energy antenna fits in and in between a small
tube that sticks out about an inch and a half and is
chromed.
It looks to me that it could be a Prandlt tube. Unfortunately
it is plugged inside at about 4 inches. Also after
checking all the static lines that come from the back,
it appears that 2 lines could be connected to it. These
two lines are also plugged somewhere.
Anybody ran into the same problem? Fixes?

[Greg Arnold]

Serge Serfaty wrote:
> On the vertical fin of the original Discus, there are
> 3 holes. one for securing the elevator, one where the
> total energy antenna fits in and in between a small
> tube that sticks out about an inch and a half and is
> chromed.
> It looks to me that it could be a Prandlt tube.

Tail mounted pitot, I believe. Though I suppose it could be used for
something else if you are using the nose pitot.

Unfortunately
> it is plugged inside at about 4 inches. Also after
> checking all the static lines that come from the back,
> it appears that 2 lines could be connected to it. These
> two lines are also plugged somewhere.
> Anybody ran into the same problem? Fixes?
>
>
>
>

[Marc Ramsey]

Serge Serfaty wrote:
> It looks to me that it could be a Prandlt tube.

My ventus B had a combination pitot/static probe at that location. It
is likely plugged by an insect that uses mud to build a nest. I've
cleared similar fittings by disconnecting the instruments, and using a
compressed air bottle to blow out the debris. Don't seal the compressed
air nozzle against the fitting, or you may blow the tubing off inside
the fin.

Marc

[Mark Zivley]

Sorry, but the compressed air idea is a poor one. Inexperience w/ this
solution is highly likely to cause a bigger problem.

1. It's very possible that the tubes come off the socket connection at
a 90 degree angle to the axis of the socket. Probably 90 degrees down.
This may explain why things only go in 4" and stop - you've hit the
back of the socket.

2. Get a flashlight and see if you can see some debris in the socket.
If so then get a piece of wire say 1/32 - 1/16 in diameter and see if
you can break it up bit by bit. Be careful around the inside wall of
the socket as there are probably a couple of o-rings in shallow grooves
which you don't want to nick or cut.

3. Use a vacuum source and suck the debris out. (Disconnect any
attached instruments first...) This won't jam the debris tighter into
place and won't blow the tubing off the back end of the fitting. If you
blow the tube off the fitting (possible w/ compressed air) you'll be
cutting a hole in the fuse to fix it.

I've never actually tried this last suggestion, but it probably is worth
considering if you think the pluggage is actually in the tubing, not the
fitting. You might consider getting some 3/32 stranded cable and work
backwards through the tube from the cockpit end. If you spin the cable
gently as you go backwards it may be possible to roto rooter the
blockage out.

No matter what you try, go slow and gentle.

Good luck.



Marc Ramsey wrote:
> Serge Serfaty wrote:
>
>> It looks to me that it could be a Prandlt tube.
>
>
> My ventus B had a combination pitot/static probe at that location. It
> is likely plugged by an insect that uses mud to build a nest. I've
> cleared similar fittings by disconnecting the instruments, and using a
> compressed air bottle to blow out the debris. Don't seal the compressed
> air nozzle against the fitting, or you may blow the tubing off inside
> the fin.
>
> Marc

[Mark Zivley]

If you de-rig daily or if you remove your probe when you hanger your
glider it should seem obvious to protect the remaining socket from
bugs/debris. On our Duo we have a short piece of tygon tubing which we
use. We connect one end of the tube to the prandtl socket and the other
end gets connected to the fin pitot. We drilled a very tiny hole in the
middle of the piece of tygon so that we don't hold any pressure between
the connections.

[Michael Huber]

> I used a small diameter drill, moderately long, and rotated
> it by hand *very* gently applying light pressure towards the
> back of the hole.

I disconnected the blocked tube at the instrument panel and had a friend
blow into it while I used a drill as described by Todd. Takes only 5
minutes, a drill and a friend, no risk of popping the tube off, no debris in
the tube and no fiddling with vacuum. If you are worried about humidity you
could use a small fan for inflating water toys or something the like (don´t
use any kind of compressor!) for blowing.

Michael